Activity detection in a star node with a plurality of coupled network nodes

ABSTRACT

The invention relates to a network comprising a plurality of network nodes and one star node, which star node is provided for the direct coupling of at least two network nodes. The star node comprises a plurality of star interfaces which are assigned to at least one network node and which comprise each an activity detector for detecting activities in the message signal coming from the assigned network node and for transferring the message signal from the assigned network node to the other star interfaces or from another star interface to the assigned network node in dependence on at least one activity.

The invention relates to a network comprising a plurality of networknodes. Such networks may be used, for example, in motor vehicles, inaerotechnics and aerospace engineering, in industrial automation (forexample, sensor systems) and domestic automation (for example, lightingtechnology, alarm systems, central heatings, climatic control, and soon).

In such a network for motor vehicle technology, for example, the TTPprotocol (TTP=Time-triggered Protocol) known from the journal“Elektronik”, no. 14, 1999, pp. 36 to 43 (Dr. Stefan Polenda, GeorgKroiss: “TTP: “Drive by Wire” in greifbarer Näahe”) may be used. Thisprotocol enables a reliable data transmission and may therefore also beused in networks for safety-related devices (for example, brakes). Inthe article mentioned above, a bus system is mentioned as a networkstructure.

It is an object of the invention to provide another network comprising aplurality of network nodes.

The object is achieved by a network of the type defined in the openingparagraph having the following characteristic features:

the network has a plurality of network nodes and one star node, whichstar node is provided for the direct coupling of at least two networknodes and includes a plurality of star interfaces which are assigned toat least one network node and which include each an activity detectorfor detecting activities in the message signal coming from the assignednetwork node and for transferring the message signal from the assignednetwork node to the other star interfaces or from another star interfaceto the assigned network node in dependence on at least one activity.

The invention relates to a network comprising a plurality of networknodes which are at least partly coupled to one another in a star node.If a network node likes to send a message, this is signaled to a starinterface in the star node. This signaling may be a certain activityfaded into the message signal of the network node, which activity callsforth a pilot signal superimposed by the message. For example, thisactivity may be performed by a change of level which the star interfaceon the assigned connection can flawlessly distinguish from the restlevel of the line.

According to the invention a star interface which is assigned to atleast one network node comprises an activity detector for detecting anactivity in the message signal of the assigned network node. First asend request is to be recognized in the assigned star interface. Thisrecognition reacts to a signal activity on the line on which the messagesignal is transported and continuously verifies whether the send nodealso further generates activities, or whether the end of the sendingactivity has been reached, respectively.

In dependence on the sequence of the activities, the message signal istransferred to the other network nodes via their assigned starinterfaces. For this purpose, for example, switchable amplifiers areconnected in the star node. In a network having high data rates and manynetwork nodes which exchange data among each other, it is necessary forthe star node to often and very rapidly perform a reconfiguration of theamplifiers in the star node. This must take place in dependence on therespectively current send node.

With the invention the requirement of a shortest possible configurationtime and a high robustness to disturbances is satisfied, whichdisturbances do not lead to an unintentional configuration.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows a network in a star structure comprising a plurality ofnetwork nodes which are coupled via an active star node,

FIG. 2 shows a basic circuit diagram of a star interface in a star node,

FIGS. 3 to 5 show various signal waveforms which may occur in the starinterface shown in FIG. 2,

FIG. 6 shows an embodiment of a star interface,

FIG. 7 shows a function diagram of an activity detector to be used inthe star interface,

FIGS. 8, 10, 12 and 15 show various embodiments of an activity detector,

FIGS. 9, 11 and 14 show various signal waveforms in the activitydetectors shown in FIGS. 8, 10 and 12, and

FIG. 13 shows a function diagram of the analog activity detector shownin FIG. 12.

An example of embodiment of the network according to the invention isshown in FIG. 1. This network comprises, for example, four network nodes1 to 4 which are coupled to one another via twisted pair lines 5 to 8provided for a symmetrical signal transmission via an active star node9. The active star node 9 performs a line adaptation so that the linepairs 5 to 8 are terminated in the active star node 9 by thecharacteristic impedance and the signals transmitted by the networknodes 1 to 4 are analyzed. If the line pairs 5 to 8 were connected toone another without the active star node 9, there would be a mismatchfor each line pair in the star node as a result of an impedance jumpfrom Z₀ to ⅓Z₀, which mismatch is caused by the respective parallelcombinations of the other line pairs.

It is also possible that instead of line pairs 5 to 8 optical fiberssuitable for the optical signal transmission are used. In that case anelectro-optical or optoelectrical converter would be necessary in thenetwork nodes 1 to 4 and in the star node 9.

The active star node 9 comprises for each line pair 5 to 8 a starinterface which enables a transfer of a message from a sending networknode to all the other network nodes connected to the active star node.The basic circuit diagram of such a star interface is shown in FIG. 2. Aline pair 5 to 8 is coupled to the inputs of a switchable amplifier 10(first switching element), which has a switching input 11, and to theoutputs of a further switchable amplifier 12 (second switching element),which has a switching input 13. On the output of the switchableamplifier 10 a signal rec_data is available and on its switching input11 a signal rec_en. To the output of the amplifier 10 is coupled anactivity detector 14, which analyzes the output signal rec_data. Theinput of the switchable amplifier 12 receives an input signal drv_data.Its switching input 13 receives the signal drv_en. The activity detector14 may also be connected upstream of the amplifier 10 if it has its ownreceiver amplifier and a switching input for switching off the activitydetection.

The activity detector 14 of a star interface is used for detectingcertain activities in a signal, which signal is applied to the assignedstar interface by the assigned network node via the assigned line pairand indicates an oncoming transmission of a message. Such activity maymean, for example, a change of the signal level in a signal with certainsubsequent signal changes. After the recognition of activity, the otherstar interfaces in the star node 9 are then switched so that theyreceive a message only from the star interface that receives a messagefrom the assigned network node. This state of the star node ismaintained until the assigned network node has completely sent itsmessage. For this purpose, continuous checks are made in the starinterface whether the message is still being sent i.e. whether activitycan still be detected on the output of the switchable amplifier 10, orwhether the sending operation was terminated (no activity). An end ofthe transmission is then recognized when no activity on the line isdetected for a defined period of time. No special signal accompanyingthe message is used for controlling the star node 9, but the transmittedmessage itself causes the activity detector 14 to maintain the onceformed star configuration (setting of the switchable amplifiers 10 and12).

If a network node wishes to send a message, it has to generate a certainactivity which is referred to as a send request. A signal waveform for asend request is shown in FIG. 3 by way of example. The signal has threephases, BI, CD and MD. By means of a level change at the end of phase BIand at the beginning of the phase CD, the network node indicates that itwishes to transmit a message. This level change is detected by theassigned star interface of the star node 9 and the switchable amplifier12 of the assigned star interface and the switchable amplifiers 11 ofthe other star interfaces are switched off and the switchable amplifiers12 of the other star interfaces are opened. If the other network nodescan receive the transmitted signal of the currently active network node,the phase MD (transmission of data) starts. The time CD which a starinterface in the star node 9 needs for detecting the signal edge,distinguishing it from a disturbance and accordingly switching its ownamplifier as well as the amplifiers of the other star interfaces, is tobe paid attention to by the transmitting network node before it isallowed to send its message. This time interval CD depends on theselected implementation of the activity detection (selected activitydetector 14) as well as the number of star nodes in the network. Thestar node 9 in FIG. 1 can be connected not only to a network node butalso to at least a further star node to which further network nodes areconnected. In that case the send request is to be transferred from thestar node 9 to the second star node and the configuration time of thissecond star node belongs to phase CD. This is necessary for the messageto be transmitted also to reach the network nodes that are connected tothe second star node.

As mentioned above, the star interface shown in FIG. 2, which detects achange of level on the link to the connected network node, is providedfor transferring this event to the other star interfaces of the starnode 9. The control signal act_det generated by the activity detector 14is used for controlling the switchable amplifiers 10 and 12. Theactivity detector 14 activates the control signal act_det after anactivity has been detected. It remains active as long as a messagetravels through the network. The changes of level inside the message arethen interpreted as an activity. If these changes fail to occur, thedetector detects that the end of the message has been reached. A signalwaveform having the phases MD and BI at the end of the message is shownby way of example in FIG. 4. The end of a message is featured by aconstant level (phase BI). The maximum time intervals of a constantlevel inside the message (phase MD) must not lead to a switching-off ofthe control signal act_det. Only after a time interval in which nochange of level has occurred for a certain period of time, is an end ofthe activity and thus the end of the message detected. The control lineact_det is then deactivated. This event is transferred to the other starinterfaces and the switched-on amplifiers of these star interfaces areswitched off. The star node again changes to a state in which it canreact to a new send request of an arbitrary network node and can providea respective configuration of the data path in the network.

FIG. 5 shows a signal waveform by way of example of a network node withnoise pulses and the control signal act_det generated therefrom of theactivity detector 14. The maximum duration (N) of a noise pulse on theline, which is to be tolerated by a system, has an influence on thedelay with which a send request is flawlessly detected. The delayT(act_det) is always larger than the tolerable noise pulse width, sothat a distinction between the two events is possible anyway. This istaken into account by the activity detector 14.

The time duration MD(max) indicates the maximum time interval betweentwo changes of level in one message. This maximum time interval dependson the selected coding and the data rate. A transmission method forwhich the time interval MD(max) cannot be determined, is unsuitable forusing the detection of the activity for the control of active starnetworks. For example, in a transmission method which utilizes the NRZcoding, no changes of level can show up for an undetermined period oftime.

The time interval T(BI) which the activity detector 14 needs to have fordetecting the end of a transmission in a reliable manner must be largerthan the maximum width MD(max) of a pulse during a message. Only in thisway is it ensured that a connection is not terminated while a message isbeing transmitted.

The time interval T(BI) is to be determined so that it contains anadditional security time interval. This reduces the probability of anerroneous detection of the end of a message transmission. This securitytime interval becomes necessary due to various system inaccuracies (forexample, scanning errors during a digital edge detection, drift ofcomponent properties, and so on).

The principle of the activity detection can generally be applied to anytype of signal transmission, for example, also to a single-linetransmission and is not restricted to a symmetrical push-pulltransmission. The decisive factor is the logic level which is applied tothe activity detector.

An example of embodiment of a star interface is shown in FIG. 6. A linepair is connected to the inputs of a switchable amplifier 15, to theoutputs of a further switchable amplifier 16, to an activity detector 17and to a terminating impedance 18. The value of the terminatingimpedance 18 corresponds to the wave resistance and is therefore usedfor the correct line termination. When the activity detector 17 detectsa send request, it generates an activated control signal which isapplied to a switch input 19 of the switchable amplifier 15, to aninverting input of the AND gate 20 and, via an amplifier 21, to a line22, which is connected to a non-inverting input of the AND gate 20. Whenthe switchable amplifier 15 is enabled, it applies data to a data line23 leading to a node. By this data line are also received data from theother star interfaces and transferred via the switchable amplifier 16 tothe assigned line pair. The non-inverting output of the AND gate 20 isconnected to a switch input 24 of the switchable amplifier 16 and, viaan inverter 25, to a switch input 26 of the activity detector 17. Anactivated control signal coming from another star interface through theinput 26 of the activity detector is used for blocking the activitydetector.

The star interface shown in FIG. 6 is connected to the other starinterfaces of a star node 9 via a wired OR combination (line 22). Theamplifier 21 is realized in FIG. 6 as an open drain amplifier. The starinterfaces of a star node 9 are in this case connected to the respectivelines 22 and 23, so that, as a result, two circuit nodes are formed. Inaddition, a resistor is provided which is coupled, on the one hand, tothe circuit node (line 22) and, on the other hand, to the logic 0 level.This resistor, together with the amplifier 21 of each star interface,forms the wired OR combination. An open collector circuit for theamplifier 21 is also possible when the logic combination is accordinglyadapted by the AND gate 20 so that the wired OR combination is realized.

The functional structure of an activity detector 14 or 17, respectively,can be learnt from FIG. 7. This FIG. 7 contains a filter 27 forsuppressing noise, an edge detector 28 and an activity detection circuit29. The signal coming in on a line pair 5 to 8 or present on the outputof a switchable amplifier is applied to the filter 27 for the noisepulse suppression. The filtered signal is analyzed by the edge detector28. This edge detector informs the activity detection circuit 29 of anedge, i.e. an edge or level change, which circuit establishes whether asend request, a message or the end of a message is present. Depending onwhether a send request, a message or the end of a message is present,the activity detection circuit 29 issues a control signal which becomesactive when there is a send request, maintains this condition when thereis a message and becomes inactive again after the end of a message hasbeen detected. The functional structure of FIG. 7 can easily be shown ina digital example of embodiment which will be described hereinafter.With an analog example of embodiment, which has also been described, thefunctional blocks of FIG. 7 cannot so clearly be assigned.

An example of embodiment of a first digital activity detector is shownin FIG. 8. All the switching elements shown in FIG. 8 need to have acommon clock (clk). The frequency of this clock signal is to be selectedsuch that a sufficient oversampling of the data signal is guaranteed. Ifthe shortest time interval of a constant level in the data stream isgiven by TB, the period of the clock signal in the star node must notexceed TB/2. The digital circuit shown in FIG. 8 includes a filter 30,an edge detector 31 and an activity detection circuit with a sendrequest memory 32 and an no-activity detection circuit 33 for detectingthe end of an activity.

The filter 30 prevents an edge, which was only generated by a briefnoise pulse on the line pair, from being interpreted as a send requestof the network node by the logic downstream in the circuit. Such afilter may comprise, for example, a shift register (for example, forthree sample values) with a downstream evaluation logic. The downstreamevaluation logic then forms part of the edge detector 31. The samplevalues pass through the shift register with always the oldest valuedropping out as soon as the new sample value is taken in. The edgedetector 31 interprets the values stored in the shift register toestablish whether a change of level (signal edge) was actually caused bya transmitting network node. When a change of level from the second tothe third sample value has taken place, an edge will not be evaluated asrecognized until also the next sample value confirms this change oflevel (sample values two and three then have the same value). If onlythe second value is different i.e. the third sample value has the samevalue as the first sample value, this is interpreted as noise by theedge detector 31 and no edge recognition is signaled.

The filter 30 and the edge detector 31 may also form a more expensivearrangement. By including more sample values, the reliability can beenhanced with which a noise signal does not lead to the activitydetection circuit being activated. However, the filter 30 and the edgedetector 31 must not unrestrictedly include many sample values for theinterpretation, because, as a result, also the time delay between theoccurrence of the signal edge and the activation of the control signalis increased.

With the aid of FIG. 9, which shows the signal waveforms in the activitydetector shown in FIG. 8, a possibility of reducing the delay caused bythe filter while the frequency of the clock (clk) remains the same isexplained in the following. The rising and falling edges of the clock(clk) are used for the signal sampling. As a result, the number ofsample values in the same time interval can be doubled. The timeinterval between the occurrence of a level change in the input signalrec_data and the occurrence of a pulse in the output signal ED of theedge detector 31 is reduced in consequence. A pulse in the output signalED is synchronized with the clock signal clk of the activity detectioncircuit and is present for one clock period.

It is to be observed that the filter 30 for the elimination of noise canalso be realized in analog form or the digital filter 30 can becomplemented by an upstream, analog filter (low-pass filter).

If an edge is recognized by the edge detector 31, i.e. its output signalED is active, this information is stored in the send request memory 32.This memory may be fed, for example, via a synchronous set input. Theoutput signal of the send request memory 32 is the control signalact_det, which—as explained above—is used for controlling its own starinterface, but also other star interfaces of the star node 9.

The send request memory keeps the control signal act_det active untilthe no-activity detection circuit 33 has established the end of theactivity and then resets the send request memory 32 via a synchronousreset input 60. The control signal act_det is then deactivated.

The no-activity detection circuit 33 verifies, after an activation bythe send request memory via a link 34, whether further level changes(=activities) occur in the output signal. These level changes are shown,as described above, by pulses in the output signal of the edge detector31. The signal rec_data in FIG. 9 contains noise N, which is recognizedas such by the edge detector and does not call forth a pulse in theoutput signal of the edge detector 31. Only when a pulse no longeroccurs for a given interval will the no-activity detection circuit 33activate its output signal. This output signal then signals that the endof the current message has been reached and resets the send requestmemory 32.

The no-activity detection circuit 33 may be a counter which, after beingtriggered, starts incrementing its internal count with an adjustablegranularity (count width). By laying down an overflow condition, a timeinterval can be defined after whose elapse the counter activates itsoutput signal. The counter may obviously also be arranged as an “elapse”counter, which starts at a predefined initial count and, when a lowerlimit (for example, zero) is fallen short of, accordingly activates itsoutput signal.

The output signal ED of the edge detector 31 is used for resetting thecount to the initial state if the no-activity detection circuit 33 isrealized as a counter. As a result, the “elapse” condition of thecounter is not reached as long as the pulses in the output signal of theedge detector 31 follow each other at brief enough intervals. Theseintervals are defined by the type of coding of the message and the datarate. Also the occurrence of the longest possible time interval in acoded message, which time interval occurs between two level changes mustnot lead to the fact that the counter reaches its “elapse” count.

A dimensioning of this interval must thus be adapted to the type ofmessage coding and the counter must be configured accordingly. This maybe effected, for example, via a programmable “elapse” count or startcount, respectively. It is also possible to set the count width for theclock (clk) by a configurable clock divider, so that the counter issupplied with an accordingly adapted count clock.

The enabling of the no-activity detection circuit 33 is effected by thesend request memory via the connection 34. As explained above, the firstedge on a line pair 5 to 8 (send request) is used for setting theswitchable amplifiers in the star interfaces for the next transmissionof messages. The send request is therefore always to be made a certaintime interval before the actual message, so that the amplifiers in thestar node 9 can be switched and connection paths in the star node aredeveloped for sending messages from one network node to the other beforethe actual message transmission commences. Therefore, it is possiblethat between the first signal edge of the signal rec_data (compare FIG.9: first pulse in the output signal ED of the edge detector 31) and thefirst signal edge caused by the message (see FIG. 9: second pulse in theoutput signal ED of the edge detector 31) a time interval elapses, whichis larger than the defined time interval of the no-activity detectioncircuit 33. The no-activity detection circuit 33 would in this caseagain terminate a connection before the message was begun by the sendingnetwork node. This may be avoided by an additional enable controlcircuit 35. This does not permit the no-activity detection circuit 33 tobe enabled via a link 36 (see FIG. 10: enable signal EN) until thesecond level change in the signal rec_data occurs.

FIG. 11 shows various signal waveforms in the activity detector shown inFIG. 10. As appears from this Figure, the enable signal EN for theno-activity detection circuit 33 will not be activated until the firstsignal edge of the data message occurs. The no-activity detectioncircuit 33 thus first probes from the beginning of the message whetherthe defined interval without an activity can be recognized in the signalrec_data.

A further example of embodiment of an analog activity detector is shownin FIG. 12. When an analog activity detector is used in the star node,no clock source is necessary. The analog activity detector receives onits input 37 the signal rec_data, which is led to the gate terminal of aP-channel MOS field effect transistor 38 and to the gate terminal of anN-channel MOS field effect transistor 39. The source terminal of thetransistor 39 is connected to ground and its drain terminal to thesource terminals of two N-channel MOS field effect transistors 40 and41. The source terminal of the transistor 38 is connected to a voltagesupply V_(CC). The drain terminal of the transistor 38 is connected tothe source terminals of two P-channel MOS field effect transistors 42and 43, whose drain terminals have node 58 in common with the drainterminals of the transistors 40 and 41, a terminal of a capacitor 44 andthe gate terminal of an N-channel MOS field effect transistor 45 and thegate terminal of a P-channel MOS field effect transistor 46. The gateterminal of the transistor 42 is connected to the gate terminal and thedrain terminal of a P-channel MOS field effect transistor 47 and to thedrain terminal of an N-channel MOS field effect transistor 48. Thesource terminal of the transistor 47 is connected to the power supplyV_(CC). The gate terminal of the transistor 48 is connected, on the onehand, to the gate terminal of the transistor 40 and, on the other hand,to a voltage source 49 (V_(ref)). The other terminal of the voltagesource 49 is connected to ground as is the source terminal of thetransistor 48.

The gate terminals of the transistors 41 and 43 and the drain terminalsof the transistors 45 and 46 form the output 50 of the analog activitydetector which delivers the control signal act_det. The source terminalof the transistor 46 is further connected to the voltage source V_(CC)and the source terminal of the transistor 45 is connected to ground asis the other terminal of the capacitor 44.

The various functions of the transistors in the analog activity detectorshown in FIG. 12 can be explained with the aid of the function diagramshown in FIG. 13. An adjustable resistor 51 is formed by the voltagesource 49 and the transistors 42, 47 and 48 and an adjustable resistor52 by the voltage source 49 and the transistor 40. The transistor 38represents a switch 53, the transistor 43 a switch 54, the transistor 51a switch 55 and the transistor 39 a switch 56. The transistors 45 and 46form an inverter 57.

In contrast to the example of embodiment of the digital activitydetector, it is assumed that on the input 37 there is a low voltagelevel (logic “0”) when no messages are transmitted. With this firststate Z1 (initial state) a low voltage level (rec_data=0, act_det=0) ispresent on the input 37 and on the output 50. The transistor 38 isturned on in this state and the transistor 39 is turned off. Since thereis also a low voltage level on the output 50, in this state thetransistor 43 is turned on and the transistor 41 is turned off. Thecapacitor 44 is charged via the transistors 38 and 43 to the positivesupply voltage V_(CC). On the node 58 between the drain terminals of thetransistors 41 and 43 the signal Z2 is present (compare FIG. 14) whichin this state has a high voltage level.

In the first state Z1, the switches 53 and 54 are closed and theswitches 55 and 56 are opened in the function diagram shown in FIG. 13.A capacitor is charged via the switches 53 and 54.

If there is a change of level on the input 37, i.e. the signal has ahigh voltage level, the transistor 38 is turned off and the transistor39 is turned on. A constant current will then flow via the drainterminal of the transistor 40, so that the capacitor 44 is discharged.

In the second state Z2 (compare FIG. 14), there are brief noise pulses.This means that a high voltage level (logic “1”) briefly occurs in thesignal rec_data. Such noise pulses are suppressed by a filter, which isdetermined by the capacitor 44 and the resistance of the transistor 40.If the noise pulses are very brief, the voltage on node 58 does notreach a switching threshold at which the output stage formed by thetransistors 45 and 46 changes state, i.e. would bring the output 50(signal act_det) to the high signal level. In consequence, the signal onthe output 50 is deactivated in the case of brief noise pulses. Afterthe end of the brief noise pulse, a low signal level on the input 37leads to the fact that the capacitor 44 is recharged very rapidly viathe turned-on transistors 38 and 43.

In the second state Z2 the switches 53 and 55 in the function diagram 13are open and the switch 56 is closed. A discharging current flows fromthe capacitor 44 through the resistor 52 and the switch 56 to ground.The capacitor 44 and the resistor 52 determine the time constant of thedischarge.

If the network node generates a send request (third state Z3), thevoltage level on the input 37 is high at least for a time intervalT(act_det) in length (compare FIG. 5, while considering the invertedinput level compared to FIG. 14). In consequence, due to the dischargevia the transistors 39 and 40, the voltage on node 58 drops below theswitching threshold of the output stage with the transistors 45 and 46(inverters). The output 50 changes to a high signal level i.e. thecontrol signal denoting a send request is activated. The transistor 43is turned off as a result and the transistor 41 is turned on. Via thetransistors 41 and 39 the capacitor 44 is suddenly completely discharged(see signal waveform of ZW). The circuit thus reaches a stable state,which indicates an activity.

With the function diagram shown in FIG. 13 the third state Z3 can beexplained as follows. After a change of the signal level on input 37 tologic “1”, the switch 53 is opened (55 remains open for the time being)and the switch 56 is closed. A discharge current flows from thecapacitor 44, through the resistor 52 and the switch 56 to ground. Afterthe switching threshold determined by the inverter 57 has been reached,the switch 54 is opened and the switch 55 is closed. As a result, thereis an amplified discharge to ground via the switches 55 and 56.

During the transmission of the actual message, there are time intervalsin which there is a low voltage level on the input 37. These timeintervals form part of the message and must therefore not lead to achange of the signal level on the output 50. As described, a low inputlevel on the node 37 leads to the fact that the transistor 38 is openedand the transistor 39 is closed. Consequently, the capacitor can nolonger discharge via the path formed by the transistors 39 and 41. Viathe transistors 38 and 42 the capacitor 44 is charged with a constantcurrent which depends on the voltage source 49 (fourth state Z4). Thetransistors 47 and 42 are to be dimensioned such that the currentflowing through 42 in the fourth state is smaller than the current thathas led to a discharge of the capacitor 44 via the transistor 40 in thesecond or third state, respectively. As a result, the duration can beset of the time interval for which a low voltage level on the input 37does not yet lead to a change of the signal level on the output 50. Theactivity detector thus remains activated as long as the time intervalshaving a low voltage level do not exceed a certain duration.

The charging operation of the capacitor 44 via the transistor 42 is thusslower than a discharge via the transistor 40. Disturbances during thedata transmission, just like the low voltage levels, do not lead to aninfluencing of the control signal act_det on the output 50. A subsequenthigh voltage level on the input 50 provides that the capacitor 44 israpidly discharged via the transistors 41 and 39 and the recognitioncircuit remains in its stable state.

The fourth state Z4 can also be explained with reference to the functiondiagram 13. The resistor 51 is selected such that, after a change from ahigh to a low voltage level on the input 37, after the switch 56 isopened and the switch 53 is closed, the capacitor 44 is slowly chargedby the voltage source V_(CC).

Only when the input signal rec_data on the input 37 continues to have anuninterruptedly low voltage level, so that as a result of the permanentcharging operation of the capacitor 44 the switching threshold of theoutput stage formed by the transistors 45 and 46 is exceeded, changesthe output signal on the output 50 to a low signal level (fifth stateZ5). This means that an end of the activity on the line was detected.

In the function diagram shown in FIG. 13 the switches 55 and 56 areopened and the switches 53 and 54 are closed in the fifth state Z5 aftera switching threshold has been exceeded. The capacitor 44 is thencharged via the switches 53 and 54.

To meet the requirements concerning a rapid reaction to a send requestand a detection of the end of a message, which detection is adapted tothe data rate and coding, the resulting resistances of the transistors40 and 42 are to be dimensioned accordingly. The resistance of thetransistor 40 (corresponds to the resistance 52 in FIG. 13), whichdetermines the response time of the detector, is smaller than theresistance of the transistor 42 (resistance 51 in FIG. 13) which isnormative for the time interval that is considered the end of theactivity.

The voltage source 49 shown in FIG. 12 may be realized by a referencevoltage source connected to a transistor diode circuit known per se.

The difference between time intervals occurring in the message andhaving a constant level and the turn-off requirement (an intervalwithout an activity, which exceeds a defined length) is to be guaranteedby the dimensioning of various transistors—as explained above. The timeintervals, however, depend on the data rate at which a transmission iseffected. To nevertheless be able to flexibly set the activity detectorshown in FIG. 12 to different data rates when the transistors have afixed dimensioning, the voltage source 49 may be arranged programmably.Thanks to the adjustable charge current the charge time constant of thecapacitor 44 can be configured. A programming of the activity detectioncircuit to a desired data rate may be realized at the voltage source 49,for example, by additional control lines for setting the referencevoltage.

The transistor 47 may also be replaced by a voltage source. Thecapacitor 44 need not be present as an individual element, but may alsobe formed by the parasitic capacitances which form the inputcapacitances of the transistors 45 and 46.

A further embodiment for the analog activity detection is shown in FIG.15. Compared to FIG. 12, the circuit includes a further P-channel fieldeffect transistor 59 whose source terminal is only connected to thedrain terminal of the transistor 43, whose gate terminal is onlyconnected to the gate terminal of the transistor 42 and whose drainterminal is only connected to the drain terminal of the transistor 41,the gate terminals of the transistors 45 and 46 and to a terminal of thecapacitor 44. Compared to FIG. 12, FIG. 15 has no connection, on the onehand, between the drain terminals of the transistors 40 and 42 and thegate terminals of the transistors 45 and 46. The transistor 59 increasesthe time constant for the charging operation of the capacitor 44 i.e. ittakes longer to increase the potential on node 58. This increases theimmunity of the circuit to noise pulses on the line.

1. A network comprising a plurality of network nodes and one star node,which star node is provided for the direct coupling of at least twonetwork nodes, and includes a plurality of star interfaces which areassigned to at least one network node and which include each an activitydetector for detecting activities in the message signal coming from theassigned network node and for transferring the message signal from theassigned network node to the other star interfaces or from another starinterface to the assigned network node in dependence on at least oneactivity, the network characterized in that each star interface furtherincludes a first and a second switching element, in that the firstswitching element in activated state passes a message from the assignednetwork node to the other star interfaces and the second switchingelement in active state passes a message from the other star interfacesto the assigned network node and in that the activity detector of a starinterface activates the first switching element and deactivates thesecond switching element when a message occurs from the assigned networknode and deactivates the first switching element and activates thesecond switching element when a message occurs from another networknode.
 2. A network as claimed in claim 1, characterized in that theactivity detector includes an edge detector for detecting an edge orflank in the message signal and an activity detection circuit forestablishing, based on the detected flank or edge, whether a sendrequest, a message or the end of a message is present.
 3. A network asclaimed in claim 2, characterized in that the activity detection circuitincludes a send request memory and an no-activity detection circuit, inthat the send request memory changes its memory contents when anactivity featuring a send request occurs, its memory contents form acontrol signal for the activation or deactivation of the switchingelements and in that the no-activity detection circuit resets the sendrequest memory after a certain period of time without the occurrence ofan activity.
 4. A network as claimed in claim 3, characterized in thatthe activity detection circuit includes an enable control signal whichenables the no-activity detection circuit after a further activityfeaturing the send request.
 5. A network as claimed in claim 1,characterized in that the activity detector includes a first and asecond switching transistor, which are arranged in dependence on themessage signal so that either the first or the second switchingtransistor is closed, and includes a capacitor which can be charged atleast via the first switching transistor and discharged via the secondswitching transistor.
 6. A network as claimed in claim 5, characterizedin that the activity detector includes a third and a fourth switchingtransistor, which are arranged in dependence on the output signal sothat either the third or the fourth switching transistor is closed,includes a first adjustable resistor connected in parallel to the thirdswitching transistor and a second adjustable resistor connected inparallel to the fourth switching transistor, and includes an invertercoupled to the output, in that during an activity featuring a sendrequest, the charged capacitor is discharged via the second and fourthswitching transistors and at the end of a message the capacitor ischarged via the first and third switching transistors.